The present invention relates generally to radiocommunication systems and, more particularly, to signal processing techniques in spread spectrum radiocommunication systems.
CDMA and spread spectrum communications have been around since the days of World War II. Early applications were predominantly military oriented. However, today there has been an increasing interest in using spread spectrum systems in commercial applications, including digital cellular radio, land mobile radio, and indoor and outdoor personal communication networks.
The cellular telephone industry has made phenomenal strides in commercial operations in the United States as well as the rest of the world. Growth in major metropolitan areas has far exceeded expectations and is outstripping system capacity. If this trend continues, the effects of rapid growth will soon reach even the smallest markets. Innovative solutions are required to meet these increasing capacity needs as well as maintain high quality service and avoid rising prices.
Throughout the world, one important step in cellular systems is to change from analog to digital transmission. Equally important is the choice of an effective digital transmission scheme for implementing the next generation cellular technology. Furthermore, it is widely believed that the first generation of Personal Communication Networks (PCNs), employing low cost, pocket-size, cordless telephones that can be carried comfortably and used to make or receive calls in the home, office, street, car, etc., will be provided by cellular carriers using the next generation digital cellular system infrastructure. The key feature demanded in these new systems is increased traffic capacity.
Currently, channel access is achieved using Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) methods. In FDMA, a communication channel is a single radio frequency band into which a signal's transmission power is concentrated. System capacity is limited by the available frequencies as well as by limitations imposed by channel reuse. In TDMA systems, a channel consists of a time slot in a periodic train of time intervals over the same frequency. System capacity is limited by the available time slots as well as by limitations imposed on channel reuse.
With FDMA or TDMA or hybrid FDMA/TDMA systems, the goal is to ensure that two potentially interfering signals do not occupy the same frequency at the same time. In contrast, Code Division Multiple Access (CDMA) allows signals to overlap in both time and frequency. Thus, all CDMA signals share the same frequency spectrum. In the frequency or the time domain, the multiple access signals appear to overlap one another.
There are a number of advantages associated with CDMA communication techniques. The capacity limits of CDMA-based cellular systems are projected to be up to twenty times that of existing analog technology as a result of the properties of a wide band CDMA system, such as improved coding gain/modulation density, voice activity gating, sectorization and reuse of the same spectrum in every cell. CDMA transmission of voice by a high bit rate decoder ensures superior, realistic voice quality. CDMA also provides for variable data rates allowing many different grades of voice quality to be offered. The scrambled signal format of CDMA completely eliminates cross talk and makes it very difficult and costly to eavesdrop or track calls, ensuring greater privacy for callers and greater immunity from air time fraud.
In a "traditional" direct-sequence CDMA system, the informational data stream to be transmitted is impressed upon a much higher rate data stream known as a signature sequence to generate a transmitted sequence. The informational data stream and the high bit rate signature sequence stream are combined by effectively multiplying the two bit streams together, assuming the binary values of the two bit streams are represented by +1 or -1. The informational data stream may consist of M'ary complex symbol values instead of binary +1 or -1 values. This combination of the higher bit rate signal with the lower bit rate data stream is called coding or spreading the informational data stream signal. Each informational data stream or channel is allocated a unique signature sequence.
Typically, the signature sequence data are binary, giving rise to stream of bits referred to as "chips." One way to generate this signature sequence is with a pseudo-noise (PN) process that appears random, but can be replicated by an authorized receiver. It is common for the period of the signature sequence to occupy one data symbol period, so that each data symbol is spread by the same Nc-chip signature sequence. A randomizing code sequence with a very long period may be added on top of this. In general, this signature sequence may be represented by real and imaginary numbers, corresponding to sending a chip value on the carrier frequency (I channel) or on a 90-degree shifted version of the carrier frequency (Q channel). Also, the signature sequence may be a composite of two sequences, where one of these sequences is a Walsh-Hadamard code word.
Typically the data symbols are binary. Thus, transmission of the signature sequence or its inverse represents one bit of information. In general, to send information symbol b using signature sequence s(n), one transmits EQU t(n)=b s(n) (1)
The receiver correlates the received signal with the known signature sequence to produce a detection statistic, which is used to detect b. For binary information symbols, when a large positive correlation results, a "0" is detected; when a large negative correlation results, a "1" is detected.
A plurality of coded information signals modulate a radio frequency carrier, for example by phase shift keying (PSK), and are jointly received as a composite signal at the receiver. Each of the spread signals overlaps all of the other spread signals, as well as noise-related signals, in both frequency and time. If the receiver is authorized, then the composite signal is correlated with one of the unique signature sequences, and the corresponding information signal can be isolated and decoded.
In the above example, a data symbol b directly modulates a signature sequence s(n), which is commonly referred to as coherent modulation. The data symbol can be binary (+1 or -1), quaternary (+1, +j, -1, -j), or, in general, M'ary, taking on any of M possible values. This allows log.sub.2 (M) information bits to be represented by one information symbol b. In another traditional CDMA modulation scheme, the information is contained in how b changes from one symbol to the next, this being referred to as differentially coherent modulation. In this case, the true information is usually given by b(t) b*(t-Ts), where * denotes complex conjugation, t is a time index, and Ts is the information symbol period. In yet another traditional CDMA modulation scheme, sometimes referred to as noncoherent modulation, an M'ary information symbol is conveyed by transmitting one of M different signature sequences.
Another CDMA technique, called "enhanced CDMA", also allows each transmitted sequence to represent more than one bit of information. A set of code words, typically orthogonal code words or bi-orthogonal code words, is used to code a group of information bits into a much longer code sequence or code symbol. A signature sequence is used to scramble the binary code sequence before transmission. This can be done by modulo-2 addition of the two binary sequences. At the receiver, the known scramble mask is used to descramble the received signal, which is then correlated to all possible code words. The code word with the largest correlation value indicates which code word was most likely sent, indicating which information bits were most likely sent. One common orthogonal code is the Walsh-Hadamard (WH) code. Enhanced CDMA can be viewed as a special case of noncoherent modulation.
In both traditional and enhanced CDMA, the "information bits" or "information symbols" referred to above can also be coded bits or symbols, where the code used is a block or convolutional code. One or more information bits can form a data symbol. Also, the signature sequence or scramble mask can be much longer than a single code sequence, in which case a subsequence of the signature sequence or scramble mask is added to the code sequence.
In many radio communication systems, the received signal includes two components: an I (in-phase) component and a Q (quadrature) component. This results because the transmitted signal has two components, and/or the intervening channel or lack of coherent carrier reference causes the transmitted signal to be divided into I and Q components. In a typical receiver using digital signal processing, the received I and Q component signals are sampled every Tc seconds, where Tc is the duration of a chip, and stored.
U.S. Pat. Nos. 5,151,919 and 5,218,619 to Paul W. Dent describe a CDMA system which allows several subscribers to communicate on the same radio frequency with a base station, which patents are incorporated here by reference. Unlike more traditional CDMA systems, interference is prevented in these patented systems by, for example, decoding signals successively in strength order from strongest to weakest, and subtracting the decoded signals from the received composite signal after decoding.
The exemplary implementations described in the foregoing incorporated patents use digital signal processing for descrambling a signal by use of its known scrambling code, transforming the signal to the spectral domain, and then notching out the spectral component associated with that signal. After notching, the remaining, nonzero components represent the transform of the other signals which have been descrambled with the first signal's code. The remainder is then transformed back to the waveform domain and the descrambling code re-applied to restore the signals to their original domain with one of them now subtracted.
In U.S. Pat. No. 5,218,619, it is disclosed that imperfect signal subtraction caused by errors in the amount of signal subtracted due to interference from other, weaker, overlapping signals may be eliminated by subtracting an already subtracted signal again in suitable amount, after having subtracted some of said other signals. This resubtraction process, referred to as reorthogonalization, can be performed by digital signal processors. However, this technique has the characteristic that the amount of processing increases with at least the fourth power of the spectrum bandwidth, making this technique costly for wideband signals.